CN109313404B - Radiation source - Google Patents

Abstract

A supercontinuum radiation source for an alignment mark measurement system comprising: a radiation source; illuminating the optical device; a plurality of waveguides; and collection optics. The radiation source is operable to generate a pulsed radiation beam. The illumination optics are arranged to receive a pulsed pump radiation beam and form a plurality of pulsed sub-beams, each pulsed sub-beam comprising a portion of the pulsed radiation beam. Each of the plurality of waveguides is arranged to receive at least one of the plurality of pulsed beamlets and broaden the spectrum of the pulsed beamlet to produce a supercontinuum beamlet. The collection optics are arranged to receive the supercontinuum sub-beams from each of the plurality of waveguides and combine them to form a supercontinuum radiation beam.

Description

Radiation source

Cross Reference to Related Applications

This application claims priority to european application 16173625.1 filed on 9/6/2016, the entire contents of which are incorporated herein by reference.

Technical Field

The present invention relates to a radiation source. More particularly, the present invention relates to a radiation source that may form part of a metrology system. For example, the radiation source may form part of an alignment system or other position measurement system within the lithographic apparatus.

Background

A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. For example, lithographic apparatus can be used in the manufacture of Integrated Circuits (ICs). In such cases, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. The pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Typically, the transfer of the pattern is performed by imaging the pattern onto a layer of radiation-sensitive material (resist) provided on the substrate. Typically, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners; in a scanner, each target portion is irradiated by scanning a pattern through a radiation beam in a given direction (the "scanning" -direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer a pattern from a patterning device to a substrate by imprinting the pattern onto the substrate.

In order to control the lithographic process of accurately placing device features on a substrate, alignment marks are typically provided on the substrate, and lithographic apparatus include one or more alignment sensors by which the positions of the alignment marks on the substrate can be accurately measured. These alignment sensors are effective position measurement devices. Different types of alignment marks and different types of alignment sensors are known, e.g. provided by different manufacturers.

There is a continuing need to provide more accurate position measurements, particularly to control overlay errors as product features become smaller and smaller.

It is an object of the present invention to provide an alternative radiation source which at least partially solves one or more problems associated with prior art radiation sources, whether or not said one or more problems are mentioned herein.

Disclosure of Invention

According to a first aspect of the present invention there is provided a supercontinuum radiation source comprising: illumination optics arranged to receive a pulsed pump radiation beam and form a plurality of pulsed beamlets, each pulsed beamlet comprising a portion of the pulsed pump radiation beam; a plurality of waveguides, each waveguide arranged to receive at least one of a plurality of pulsed beamlets and broaden the spectrum of the pulsed beamlets so as to produce supercontinuum beamlets; and collecting optics arranged to receive the supercontinuum sub-beams from each of the plurality of waveguides and combine them to form a supercontinuum radiation beam.

Supercontinuum generation is the formation of a broad continuous spectrum (wavelength range of about 400nm to 2500nm) by the propagation of short, high power pulses through a nonlinear medium. The term supercontinuum does not cover a specific phenomenon but a number of non-linear effects leading to considerable spectral broadening of the light pulses. The nonlinear effects involved depend on the dispersion in the material and account for effects such as self-phase modulation, raman scattering, phase matching and solitons. Current supercontinuum fiber lasers include high peak power pump sources to efficiently initiate nonlinear effects in the fiber and a specific length of photonic crystal fiber for supercontinuum generation.

The supercontinuum radiation source may be adapted for use in an optical measurement system, such as an alignment mark measurement system or a semiconductor inspection apparatus in general. Furthermore, the supercontinuum radiation source according to the invention can be advantageously applied in other technical fields than the field of lithography, such as medical tomography, measurement of optical fiber or component attenuation, interferometry or spectroscopy, optical coherence tomography, confocal microscopy, nanotechnology, biomedicine, consumer electronics, etc.

A first aspect of the invention provides a radiation source having a broad spectrum that is particularly useful for alignment mark measurement systems. When a pulse of a pump radiation beam propagates through the waveguide, a supercontinuum is formed due to various nonlinear optical effects. It should be understood that the term "waveguide" as used herein refers to a structure or medium configured to guide waves, particularly electromagnetic waves. Such a waveguide may form part of an integrated optical system, i.e. it may be provided "on-chip". Alternatively, such a waveguide may be a free space waveguide. Free space waveguides include a variety of different types of optical fibers including, for example, photonic crystal fibers.

Due to the inherent non-linear nature of these effects, supercontinuum radiation sources typically suffer from spectral noise, pulse-to-pulse fluctuations and unstable output modes, even if the pump radiation source is stable (in which its output has substantially no pulse-to-pulse variations).

A first aspect of the invention provides an arrangement in which a plurality of supercontinuums are generated (one in each of a plurality of waveguides) and the supercontinuums are superimposed (by collection optics). This arrangement is advantageous over prior art arrangements because noise and pulse-to-pulse variations within different individual supercontinuums will at least partially cancel each other out. Thus, the arrangement provides a broad spectrum radiation source of the type suitable for use in an alignment mark measurement system, which requires a more stable output than prior art arrangements to allow measurements made using such an alignment mark measurement system to be of sufficiently high accuracy.

Generally, a waveguide will be able to support radiation if the radiation intensity (i.e., power per unit area) is below the threshold of the waveguide. If radiation with an intensity above the threshold is coupled into the waveguide, the waveguide may be damaged. The first aspect of the present invention allows the power of a pulsed pump radiation beam to be distributed over a plurality of waveguides by splitting the pulsed pump radiation beam into a plurality of pulsed sub-beams, each of which propagates through a different waveguide to produce a supercontinuum. Thus, a passive coupling of the pulsed pump radiation beam into the plurality of waveguides is provided (i.e. no (optical) amplification is applied). This means that for a given desired output power of the source, the cross-sectional area of each of the plurality of waveguides can be reduced relative to the cross-sectional area of a single waveguide in a prior art supercontinuum source. In particular, in some embodiments, the size of the waveguide may be sufficiently reduced so that the waveguide may include integrated optics, even for relatively bright radiation sources (e.g., having power on the order of 1W or more). That is, the waveguide may be disposed on a chip (e.g., as an integrated optical system) and may be formed using semiconductor fabrication techniques. The noise and pulse-to-pulse variation of the supercontinuum source depend on many factors, one of the main factors being the length of interaction that the nonlinear processes leading to the generation of the supercontinuum can act on. Such on-chip waveguides can have a small interaction length over which the nonlinear processes that result in supercontinuum generation can work, rather than, for example, free-space waveguides (e.g., photonic crystal fibers) that are used to generate supercontinuum. This, in turn, reduces the noise and pulse-to-pulse variation of the supercontinuum produced by each of the multiple waveguides relative to the noise and pulse-to-pulse variation of a single waveguide in prior art supercontinuum sources.

Thus, the radiation source according to the first aspect of the invention allows an improvement in both noise and pulse-to-pulse variation of the radiation source. Each individual supercontinuum can be generated with a more stable output (for a given total output power) than prior art arrangements, and in addition, multiple supercontinuums are combined to at least partially average noise and pulse-to-pulse fluctuations.

Another advantage of the first aspect of the invention is that the supercontinuum radiation source has a certain level of redundancy and can still operate to some extent even in the event of failure of one of the plurality of waveguides.

The plurality of waveguides may include integrated optics. That is, the waveguide may be disposed on a chip (e.g., as an integrated optical system) and may be formed using semiconductor fabrication techniques. Such on-chip waveguide passlengths have small interaction lengths over which nonlinear processes that result in supercontinuum generation can work, rather than, for example, free-space waveguides (e.g., photonic crystal fibers) that are used to generate supercontinuum. This, in turn, reduces the noise and pulse-to-pulse variation of the supercontinuum produced by each of the multiple waveguides relative to the noise and pulse-to-pulse variation of a single waveguide in prior art supercontinuum sources. Furthermore, the short interaction length, compact size and mature manufacturing technology of the integrating optics allow the supercontinuum radiation source to benefit from better mode control and polarization control than the supercontinuum radiation beam output by the supercontinuum radiation source.

The plurality of waveguides are formed of silicon nitride (Si)₃N₄) Formed and coated with a material or silicon orSilicon dioxide (SiO)₂) And (4) surrounding.

The plurality of waveguides may be formed on a common substrate.

Each of the plurality of waveguides may have a width on the order of 1 μm or less and a height on the order of 500nm or less. Each of the plurality of waveguides has a length of 10nm or less.

The supercontinuum radiation beam has a power of at least 1W. Known supercontinuum radiation sources with output powers of this order are feasible by using, for example, photonic crystal fibers as the nonlinear optical medium. The present invention may allow a relatively bright supercontinuum radiation source (i.e. having an output power of at least 1W) to be formed from integrated optics by providing a plurality of supercontinuum producing waveguides. Furthermore, the supercontinuum radiation source according to embodiments of the present invention provides a very compact arrangement, which is particularly significantly smaller than known supercontinuum radiation sources based on photonic crystal fibres.

The supercontinuum radiation beam may have a spectrum comprising radiation in the wavelength range 400 to 2600 nm. This includes radiation from visible to far infrared light. Thus, the supercontinuum radiation beam may have a bandwidth of about 500 THz.

The supercontinuum radiation may include 100 or more waveguides.

The illumination optics and/or the collection optics may be implemented by a waveguide system.

The illumination optics and/or collection optics may include a plurality of sets of waveguides ordered in a sequence, and the waveguides from each set of waveguides are optically coupled to a plurality of waveguides in a next set of waveguides in the sequence.

The illumination optics and/or collection optics include a plurality of lensing fibers, each coupled to at least one of the plurality of waveguides.

The illumination optics and/or the collection optics may be implemented by a lens system.

The illumination optics may include first optics and focusing optics. The first optic may be arranged to receive a radiation beam from the radiation source and direct it to the focusing optic. The focusing optics may be arranged to optically couple different portions of the beam of pump radiation to at least two of the plurality of waveguides.

The focusing optics may comprise an array of focusing lenses, each focusing lens being arranged to focus a different portion of the beam of pump radiation to a focal point at or near the entrance of one of the plurality of waveguides.

The supercontinuum radiation beam may comprise a single mode.

The illumination optics and collection optics may include a combination of integrating optics and free space optics.

According to a second aspect of the present invention, there is provided an optical measurement system comprising a supercontinuum radiation source according to the first aspect of the present invention.

According to a third aspect of the present invention, there is provided an alignment mark measurement system comprising: the supercontinuum radiation source of any one of the preceding embodiments; an optical system operable to project the supercontinuum radiation beam onto an alignment mark located on a substrate provided on a substrate table; a sensor operable to detect radiation diffracted/scattered by an alignment mark and to output a signal containing information relating to the position of the alignment mark; and a processor configured to receive signals from the sensors and to determine a position of the alignment mark relative to the substrate table in dependence on the signals.

According to a fourth aspect of the invention, there is provided a lithographic apparatus comprising an alignment mark measurement system according to the third aspect of the invention.

It will be apparent to those skilled in the art that various aspects and features of the invention set forth above or below may be combined with various other aspects and features of the invention.

Drawings

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

FIG. 1A schematically depicts a lithographic system that includes an alignment system according to an embodiment of the invention;

FIG. 1B shows a plan view of a substrate W which may represent either of the two substrates of FIG. 1A;

FIG. 1C depicts a plan view of a patterning device that may be used by the lithography system of FIG. 1A;

FIG. 2, which includes FIGS. 2(a) and 2(b), schematically depicts various forms of alignment marks that may be provided on a substrate in the apparatus of FIG. 1;

FIG. 3 is a schematic block diagram of a first alignment sensor scanning alignment marks in the apparatus of FIG. 1;

FIG. 4 is a schematic diagram of a second alignment sensor that may be used as the alignment sensor in the device of FIG. 1, including off-axis illumination and an optional asymmetry measurement arrangement (not shown in detail), and also showing features of multiple wavelengths and polarizations;

FIG. 5 is a schematic diagram of a supercontinuum radiation source that may form part of the alignment sensor of FIGS. 3 and 4, according to an embodiment of the present invention;

figure 6a is a cross-sectional view of a portion of a waveguide provided as part of an integrated optical system in a plane (xy-plane) perpendicular to the optical axis (z-direction) of the waveguide along which, in use, radiation propagates through the waveguide;

FIG. 6b shows a partial cross-sectional perspective view of a portion of the waveguide 650 shown in FIG. 6a, with cladding material not shown;

FIG. 7 shows a first embodiment of the supercontinuum radiation source of FIG. 5;

FIGS. 8a and 8b show two variations of the second embodiment of the supercontinuum radiation source of FIG. 5; and

fig. 9a and 9b show two variants of the third embodiment of the supercontinuum radiation source of fig. 5.

Detailed Description

Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of integrated circuits, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, Liquid Crystal Displays (LCDs), thin-film magnetic heads, etc. Those skilled in the art will appreciate that, in the context of such alternative applications, any use of the terms "wafer" or "die" herein may be considered as synonymous with the more general terms "substrate" or "target portion", respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. In addition, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

The term "patterning device" used herein should be broadly interpreted as referring to a device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, attenuated phase-shift, and various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions; in this way, the reflected beam is patterned.

The support structure holds a patterning device. The support structure holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure may use mechanical clamping, vacuum, or other clamping techniques (e.g., electrostatic clamping under vacuum conditions). The support structure may be a frame or a table, for example, which may be fixed or movable as required and which may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms "reticle" or "mask" herein may be considered synonymous with the more general term "patterning device".

The term "projection system" used herein should be broadly interpreted as encompassing various types of projection system, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate, for example, for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein may be considered as synonymous with the more general term "projection system".

The term "illumination system" as used herein may encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling a beam of radiation, and such components may also be referred to below, collectively or singularly, as a "lens".

The lithographic apparatus may also be of a type: wherein the substrate is immersed in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.

FIG. 1A schematically depicts a lithographic apparatus according to a particular embodiment of the invention. The lithographic apparatus comprises:

an illumination system (illuminator) IL configured to condition a radiation beam PB (e.g. a UV radiation beam or a DUV radiation beam);

a frame MF;

a support structure (e.g. a mask table) MT to support a patterning device MA;

two substrate tables (e.g. wafer tables) WT1, WT2, each for holding a substrate (e.g. a resist coated wafer) W1, W2, respectively; and

a projection system (e.g. a refractive projection lens) PL configured to image a pattern imparted to the radiation beam PB by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W held by one of the two substrate tables WT1, WT 2.

The frame MF is a vibration-isolating frame that substantially isolates external influences such as vibrations. For example, the frame MF may be supported by a pedestal (not shown) on the ground via an acoustic damping mount (not shown) in order to isolate the frame MF from vibrations of the pedestal. These acoustic damping mounts can be actively controlled to isolate vibrations introduced by the pedestal and/or by the isolation frame MF itself. In the dual stage lithographic apparatus shown in fig. 1A, the alignment system AS and the topography measurement system TMS are arranged on the left-hand side and the projection system PL is arranged on the right-hand side. The projection system PL, the alignment system AS and the topography measurement system TMS are connected to an isolation frame MF.

The support structure MT is movably mounted to a frame MF via a first positioning device PM. The first positioning device PM can be used to move the patterning device MA and accurately position the patterning device MA with respect to the frame MF (and the projection system PL coupled to the frame MF).

Substrate tables WT1, WT2 are movably mounted to a frame MF via a first substrate positioner PW1 and a second substrate positioner PW2, respectively. The first and second substrate positioning devices PW1, PW2 can be used to move substrates W1, W2, which are held by substrate tables WT1, WT2, respectively, and accurately position substrates W1, W2 with respect to the frame MF (and projection system PL, alignment system AS and topography measurement system TMS, which are connected to frame MF). The support structure MT and the substrate tables WT1, WT2 may be referred to collectively as an object table. The first and second substrate positioning devices PW1, 2 may each be considered to be a scanning mechanism operable to move the substrate table WT1, WT2 relative to the radiation beam along a scan path such that the radiation beam is scanned across a target portion C of the substrate W.

Thus, the lithographic apparatus shown in FIG. 1A is of a type having two substrate tables WT1, WT2, which may be referred to as a dual stage apparatus. In these "multi-stage" machines the two substrate tables WT1, WT2 are used in parallel, with one of the substrate tables being used for exposure while a preparatory step is carried out on the other substrate table. The preliminary steps may include mapping the surface of the substrate using the level sensor LS and measuring the position of the alignment marks on the substrate by using the alignment sensor AS. This enables a significant increase in the throughput of the device. IF the position sensor IF is not capable of measuring the position of the substrate table while it is at the measurement station and at the exposure station, a second position sensor may be provided to enable tracking of the position of the substrate table at both stations.

In FIG. 1A, the substrate table WT1 is provided on the left and the substrate table WT2 is provided on the right. In such a configuration, the substrate table WT1 can be used to perform various preparatory steps with respect to the substrate W1, using the alignment system AS (described more fully below) and the topography metrology system TMS, before exposure of the substrate W1 held thereby. Meanwhile, substrate table WT2 may be used to expose another substrate W2 held by substrate table WT 2. Once the substrate W2 held by the substrate table WT2 has been exposed and preparatory steps relating to the substrate W1 held by the substrate table WT1 have been carried out, the positions of the two substrate tables WT1, WT2 are swapped. Subsequently, the substrate W1 to be held by the substrate table WT1 is exposed to radiation, and the substrate W2 held by the substrate table WT2, which has been exposed to radiation before, is replaced with a new substrate, and various preparatory steps relating to the new substrate are carried out.

Thus, each of the two substrate tables WT1, WT2 may be disposed to the left or right of fig. 1A. Unless otherwise noted, in the following, the substrate table WT1 will generally represent the substrate table then being disposed to the left, and the substrate table WT2 will generally represent the substrate table then being disposed to the right.

Fig. 1B shows a plan view of a substrate W which may represent either one of the two substrates W1, W2 of fig. 1A. In the following, the left and right substrates of the lithographic apparatus will be referred to as substrates W unless otherwise indicated. Fig. 1C shows a plan view of the patterning device MA, which is provided with patterning device alignment marks (schematically shown as blocks M1, M2).

As shown here, the apparatus is of a transmissive type (e.g. employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g. employing a programmable mirror array of a type as referred to above).

The illuminator IL receives a radiation beam from a radiation source SO. For example, when the source SO is an excimer laser, the source SO and the lithographic apparatus may be separate entities. In such cases, the source SO is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the apparatus, for example when the source is a mercury lamp. The illuminator IL may be referred to as a radiation system. Alternatively, the source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

The illuminator IL may vary the intensity distribution of the beam. The illuminator may be arranged to limit the radial extent of the radiation beam such that the intensity distribution is non-zero in an annular region in a pupil plane of the illuminator IL. Additionally or alternatively, the illuminator IL may be further operable to limit the distribution of the beam in the pupil plane such that the intensity distribution is non-zero in a plurality of equally spaced sectors in the pupil plane. The intensity distribution of the radiation beam in a pupil plane of the illuminator IL may be referred to as an illumination mode.

The illuminator IL may comprise an adjusting device AM for adjusting the intensity distribution of the beam. Generally, at least an outer radial extent and/or an inner radial extent (commonly referred to as σ -outer and σ -inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. The illuminator IL may also be operable to vary the angular distribution of the beam in a pupil plane of the illuminator. For example, the illuminator IL may be operable to vary the number and angular extent of sectors in a pupil plane in which the intensity distribution is non-zero. By adjusting the intensity distribution of the beam in a pupil plane of the illuminator, different illumination modes can be achieved. For example, the intensity distribution can have a multipole distribution, such as, for example, a dipole, quadrupole, or hexapole distribution, by limiting the radial extent and angular extent of the intensity distribution in a pupil plane of the illuminator IL, as is known in the art. The desired illumination mode may be obtained by inserting optical devices that provide the illumination mode into the illuminator IL.

The illuminator IL may be operable to change the polarization of the beam and may be operable to adjust the polarization by using an adjusting device AM. The polarization state of the radiation beam across a pupil plane of the illuminator IL may be referred to as a polarization mode. The use of different polarization modes may allow a greater contrast to be achieved in the image formed on the substrate W. The radiation beam may be unpolarized. Alternatively, the illuminator IL may be arranged to linearly polarize the radiation beam. The polarization direction of the radiation beam may vary across a pupil plane of the illuminator IL, i.e., the polarization direction of the radiation may be different in different regions of the pupil plane of the illuminator IL. The polarization state of the radiation may be selected depending on the illumination mode.

In addition, the illuminator IL generally includes various other components, such as an integrator JN and a condenser CO. The illuminator IL provides a conditioned radiation beam PB having a desired uniformity and intensity distribution in its cross-section.

The shape and (spatial) intensity distribution of the conditioned radiation beam PB is defined by the optics of the illuminator IL. In scan mode, the conditioned radiation beam PB can be such that it forms a substantially rectangular band of radiation on the patterning device MA. The band of radiation may be referred to as an exposure slit (or slit). The slits may have a longer dimension (which may be referred to as the length of the slit) and a shorter dimension (which may be referred to as the width of the slit). The width of the slit may correspond to the scanning direction (y direction in fig. 1), and the length of the slit may correspond to the non-scanning direction (x direction in fig. 1). In scan mode, the length of the slit limits the extent of the target portion C in the non-scanning direction that can be exposed in a single dynamic exposure. In contrast, the extent in the scan direction of the target portion C that can be exposed in a single dynamic exposure is determined by the length of the scanning motion.

The terms "slit", "exposure slit" or "band or radiation" may be used interchangeably to refer to a band of radiation generated by the illuminator IL in a plane perpendicular to the optical axis of the lithographic apparatus. The plane may be at or near the patterning device MA or the substrate W. The terms "slit profile", "profile of the radiation beam", "intensity profile" and "profile" may be used interchangeably to denote the shape of the (spatial) intensity distribution of the slit, in particular in the scan direction.

The illuminator IL includes two masking blades (shown schematically in fig. 1A and 1B). Each of the two masking blades is substantially parallel to the length of the slit, the two masking blades being disposed on opposite sides of the slit. Each of the masking blades is independently movable between a retracted position in which the masking blade is not disposed in the path of the radiation beam PB and an inserted position; in the inserted position, the shield blade blocks the radiation beam PB. The masking blade is disposed in a field plane of the illuminator IL. Thus, by moving the masking blade into the path of the radiation beam, the profile of the radiation beam PB can be abruptly truncated, thus limiting the extent of the field of the radiation beam PB in the scan direction. The masking blade may be used to control which portions of the exposure area receive radiation.

The patterning device MA is also arranged in the field plane of the lithographic apparatus. In one embodiment, the masking blade may be disposed adjacent to the patterning device MA such that both the masking blade and the patterning device MA are substantially in the same plane. Alternatively, the masking blade may be separate from the patterning device MA so that they are each in a different field plane of the lithographic apparatus, and suitable focusing optics (not shown) may be provided between the masking blade and the patterning device MA.

The illuminator IL includes an intensity adjuster IA (shown schematically in FIG. 1A). The intensity adjuster IA is operable to attenuate the radiation beam on opposite sides of the radiation beam, as now described. The intensity adjuster IA comprises a plurality of movable fingers arranged in pairs, each pair comprising one finger on each side of the slit (i.e. each pair of fingers is separated in the y-direction). The pairs of fingers are arranged along the length of the slot (i.e., extending in the x-direction). Each movable finger is independently movable in the scan direction (y-direction). That is, the fingers may be movable in a direction perpendicular to the length of the slot. In use, each moveable finger is independently moveable in the scanning direction. For example, each moveable finger may be moveable between at least a retracted position in which the moveable finger is not disposed in the path of the radiation beam and an inserted position; in the inserted position, the moveable finger partially blocks the radiation beam. By moving the fingers, the shape and/or intensity distribution of the slit can be adjusted.

The field may be in the penumbra of the fingers so that the fingers do not abruptly stop the radiation beam PB. The pairs of fingers may be used to apply different levels of attenuation of the radiation beam PB along the length of the slit.

For example, the fingers may be used to ensure that the integral of the intensity profile of the radiation beam PB across the width of the slit is substantially constant along the length of the slit.

The radiation beam PB exiting the illuminator IL is incident on a patterning device (e.g. a mask) MA, which is held on the support structure MT. Having traversed the patterning device MA, the beam PB passes through the projection system PL, which focuses the beam onto a target portion C of the substrate W. With the aid of the second substrate positioner PW2 and position sensor IF (e.g. an interferometric device), the substrate table WT2 can be moved accurately, e.g. so as to position different target portions C in the path of the beam PB, relative to the frame MF. Similarly, the first positioner PM and another position sensor (which is not explicitly depicted in fig. 1A) may be used to accurately position the patterning device MA with respect to the frame MF, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the object tables MT and WT1 and WT2 will be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the positioning devices PM, PW1 and PW 2. Patterning device MA and substrate W may be aligned using patterning device alignment marks M1, M2 and substrate alignment marks P1, P2.

Projection system PL may apply a demagnification factor to radiation beam PB to form an image having features smaller than corresponding features on patterning device MA. For example, a reduction factor of 4 may be applied.

In scan mode, the first positioning device PM is operable to move the support structure MT with respect to the radiation beam PB, which has been conditioned along a scan path by the illuminator IL. In one embodiment, the support structure MT is scanned at a constant scanning speed V_MTLinearly moving in the scanning direction. As described above, the slit is oriented such that its width extends in the scan direction (which coincides with the y-direction of fig. 1). In any event, each point on the patterning device MA illuminated by a slit will be imaged by the projection system PL onto a single conjugate point in the plane of the substrate W. As the support structure MT moves in the scan direction, the pattern on the patterning device MA moves across the width of the slit at the same speed as the support structure MT. In particular, each point on the patterning device MA is at a speed V_MTMoving across the width of the slit in the scan direction. Due to the movement of the support structure MT, a conjugate point in the plane of the substrate W, corresponding to each point on the patterning device MA, will move relative to the slit in the plane of the substrate table WT 2.

To form an image of the patterning device MA on the substrate W, the substrate table WT2 is moved such that each point on the patterning device MA remains stationary with respect to the substrate W at a conjugate point in the plane of the substrate W. The velocity (both magnitude and direction) of the substrate table WT2 relative to the projection system PL is determined by the demagnification and image reversal characteristics (in the scan direction) of the projection system PL. In particular, if the characteristics of projection system PL are such that the image of patterning device MA formed in the plane of substrate W is reversed in the scan direction, substrate table WT2 should be moved in the opposite direction to that of support structure MT. That is, the motion of substrate table WT2 should be anti-parallel to the motion of support structure MT. In addition, if the projection system PL applies a demagnification factor α to the radiation beam PB, the distance travelled by each conjugate point in a given time period will be a times less than the distance travelled by the corresponding point on the patterning device. Thus, the magnitude of the velocity of the substrate table WT2 is non-zeroV_MTI should be V_MT|/α。

During exposure of target portion C, a masking blade of illuminator IL may be used to control the width of the slit of radiation beam PB, which in turn limits the extent of the exposure area in the plane of patterning device MA and substrate W, respectively. That is, the masking blade of the illuminator serves as a field stop for the lithographic apparatus.

In the case of a scanning mode, the lithographic apparatus is operable to expose a target portion C of the substrate W having a substantially fixed area to radiation. For example, the target portion C may comprise a portion of a die, one or several dies. A single wafer may be exposed to radiation in a plurality of steps, each step involving exposure of a target portion C, followed by movement of the substrate W. After exposure of a first target portion C, the lithographic apparatus may be operable to move the substrate W relative to the projection system PL such that another target portion C can be exposed to the radiation. For example, between exposures of two different target portions C on a substrate W, substrate table WT2 may be operable to move the substrate W so as to position the next target portion in preparation for being scanned across the exposure area.

Alternatively, the depicted apparatus can be used in another mode, in which the support structure MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT2 is moved or scanned while a pattern imparted to the beam PB is projected onto a target portion C. In this mode, a pulsed radiation source is typically employed, and the programmable patterning device is updated as required after each movement of the substrate table WT2 or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

AS described further below, the alignment system AS measures the position of alignment marks (schematically depicted by blocks P1, P2 in fig. 1B) provided on a substrate W held on the left-hand substrate table WT 1. In addition, the topography measurement system TMS is used to measure the topography of the surface of the substrate W held on the left hand substrate table WT 1. The first substrate positioner PW1 and position sensors (which are not explicitly depicted in fig. 1A) can be used to accurately position the substrate table WT1 with respect to the frame MF (and the alignment system AS and the topography measurement system TMS connected thereto).

The topography measurement system TMS may be operable to output a signal s indicative of the height of the substrate W1₁. The alignment sensor AS may be operable to output a signal s indicative of the position of one or more alignment marks on the substrate W1 or substrate table WT1₂. Output signal s₁、s₂Is received by the processor PR.

Signal s output by topography measurement system TMS₁May be analyzed by the processor PR to determine the height of the substrate W1. The processor PR may be used to generate a map of the topography of the substrate W1. The processor PR may include a memory and may be operable to store information relating to the topography of the entire substrate W1. The topography of the surface of the substrate W1 may be referred to as a height map. During exposure of the substrate W (on the right hand side of FIG. 1A), it is desirable to maintain the substrate W in the focal plane of the projection system PL. To achieve this, the substrate table WT2 can be moved in the z direction, the movement of the substrate table WT2 being determined in dependence on the topography of the surface of the substrate W (as previously determined by the topography measurement system TMS).

Signal s output by alignment sensor AS₂May be analyzed by the processor PR to determine the position of one or more alignment marks on the substrate W1 and substrate table WT 1. The first substrate positioner PW1 may be operable to move the substrate table WT1 so AS to position each alignment mark in turn beneath an alignment sensor AS, while a position sensor IF (position sensor IF or another position sensor dedicated to the measurement station) measures the substrate table WT 1. As an initial step, the first substrate positioning device PW1 may be used to position one or more alignment marks on the substrate table WT1 under the alignment sensor AS and to determine the position of each alignment mark. Subsequently, the first substrate positioner PW1 may be used to align one or more of the substrates W1The marks are positioned below the alignment sensor AS and the position of each alignment mark is determined. For example, the position of the substrate table WT1 AS determined by the position sensor may be recorded with each alignment mark directly below the alignment sensor AS. Effectively, measuring the position of the alignment marks on the substrate table WT1 allows the position of the substrate table WT1 (relative to the frame MF to which the alignment system AS is connected) to be calibrated AS determined by a position sensor (e.g. sensor IF). Measuring the position of alignment marks on the substrate W1 allows the position of the substrate W1 relative to the substrate table WT1 to be determined.

The processor PR may be considered a digital signal processing system. The processor PR may comprise, for example, one or more microprocessors or one or more Field Programmable Gate Arrays (FPGAs), or the like.

In addition to the data from the alignment system AS and the topography measurement system TMS, the processor PR also receives position information (see signal s in FIG. 1A) from the first substrate positioner PW1 and/or the substrate table WT1 from a position sensor (e.g. sensor IF)₃). Since the substrate is fixed to the substrate table WT1 (typically via clamps), position information relating to the substrate table WT1 can be converted into position information relating to the substrate W using information from the alignment sensor AS.

The apparatus may comprise a lithographic apparatus control unit (not shown) which controls all movements and measurements of the various actuators and sensors described. The lithographic apparatus control unit may include signal processing and data processing capabilities to perform desired calculations related to the operation of the apparatus. The processor PR may form part of a control unit of the lithographic apparatus. In practice, the lithographic apparatus control unit may be implemented as a system of many sub-units, each of which handles the real-time data acquisition, processing and control of subsystems or components within the apparatus. For example, one processing subsystem may be dedicated to servo-controlling the first and second substrate positioning devices PW1 and PW 2. The separate units may even handle coarse and fine actuators, or different shafts. Another unit may be dedicated to reading out the position sensor IF (and, IF used, another position sensor for the measuring station). Overall control of the apparatus may be controlled by the central processing unit, in communication with the subsystem processing units, with operators, and with other apparatus involved in the lithographic manufacturing process.

Fig. 2(a) shows examples of alignment marks 202, 204 provided on the substrate W for measuring the X position and the Y position, respectively. Each alignment mark in this example comprises a series of bars formed in a product layer or other layer applied to or etched into the substrate. These bars are regularly spaced and act as grating lines so that the alignment marks can be regarded as diffraction gratings with a sufficiently well-known spatial period (pitch). The bars on the X-direction alignment marks 202 are parallel to the Y-axis to provide periodicity in the X-direction, and the bars on the Y-direction alignment marks 204 are parallel to the X-axis to provide periodicity in the Y-direction. The alignment sensor AS (shown in fig. 1) optically scans each alignment mark with spots of radiation 206 (X-direction), 208 (Y-direction) to obtain a periodically varying signal, e.g. a sine wave. The phase of this signal is analysed to measure the position of the alignment marks relative to the alignment sensor, and thus the substrate W position relative to the alignment sensor, which is subsequently fixed relative to the frame MF of the apparatus. The scanning motion is schematically indicated by a wide arrow, the progressive position of the spot 206 or 208 being indicated by a dashed line. The pitch of the bars (grating lines) in the alignment pattern is typically much larger than the pitch of the product features to be formed on the substrate, and the alignment sensor AS uses a much longer wavelength (or typically multiple wavelengths) of radiation than the exposure radiation used to apply the pattern to the substrate. However, fine position information can be obtained because a large number of bars allows the phase of the repetitive signal to be accurately measured.

Coarse and fine marks may be provided so that the alignment sensor can distinguish between different cycles of the periodic signal and the exact position (phase) within a cycle. Alignment marks with different pitches can also be used for this purpose. These techniques are again well known to those skilled in the art and will not be described in detail herein. The design and operation of such sensors is well known in the art, and each lithographic apparatus may have its own sensor design. The alignment sensor AS may typically be of the form described in US patent US 69661116 (den Boef et al). Fig. 2(b) shows a modified mark for use with a similar alignment system, the X and Y positions of which can be obtained by a single optical scan using illumination spots 206 or 208. The alignment mark 210 has bars arranged at 45 degrees with respect to both the X-axis and the Y-axis. The combined X and Y measurements may be performed using techniques described in, for example, published patent application US 2009/195768.

Fig. 3 is a schematic block diagram of a known alignment sensor AS. The illumination source 220 provides a radiation beam 222 having one or more wavelengths, which radiation beam 222 is diverted by a spot mirror 223 through an objective lens 224 onto an alignment mark (such as alignment mark 202) on the substrate W. As schematically shown in fig. 2, in the example of the alignment sensor of the present invention based on US 69661116 mentioned above, the diameter of the illumination spot 206 used to illuminate the alignment mark 202 may be slightly smaller than the width of the alignment mark itself.

Radiation scattered by the alignment mark 202 is acquired by the objective lens 224 and collimated into an information carrying beam 226. The self-referencing interferometer 228 is of the type disclosed in US 69661116 mentioned above, processes the beam 226 and outputs a separate beam (for each wavelength) onto the sensor array 230. The spot mirror 223 then conveniently acts as a zero-order stop, so that the information carrying beam 226 comprises only the higher-order diffracted radiation from the alignment mark 202 (this is not necessary for the measurement, but this improves the signal-to-noise ratio). The intensity signals 232 from the individual sensors in the sensor grid 230 are provided to a processing unit PU, which may form part of the processor PR of fig. 1. The values of the X-position and the Y-position on the substrate with respect to the reference frame RF are output by a combination of the optical processing in block 228 and the calculation processing in unit PU. The processing unit PU may be separate from the control unit LACU shown in fig. 1, or the processing unit PU and the control unit LACU may share the same processing hardware for design choice and convenience. In the case where unit PU is separate, part of the signal processing may be performed in unit PU and another part of the signal processing may be performed in unit LACU.

As already mentioned, a single measurement of the illustrated type only fixes the position of the alignment marks within a certain range corresponding to one pitch of the alignment marks. A coarser measurement technique may be used in conjunction with the single measurement to identify which period of the sine wave is the period containing the marked location. To increase accuracy and/or to robustly measure the alignment marks, regardless of the material from which the alignment marks are made and on which materials the alignment marks are located above or below, the same process at a coarser and/or finer level may be repeated at different wavelengths. The wavelengths may be multiplexed and demultiplexed optically to process the wavelengths simultaneously and/or the wavelengths may be multiplexed using time or frequency division. Examples in the present disclosure will utilize measurements at several wavelengths to provide a practical and robust measurement device (alignment sensor) with reduced sensitivity to alignment mark asymmetry.

Referring in more detail to the measurement process, marked V in FIG. 3_wThe arrow of (a) illustrates the scan speed of the spot 206 across the length L of the alignment mark 202. In this example, the alignment sensor AS and spot 206 remain substantially stationary while the substrate W is at a velocity V_wAnd (4) moving. Thus, the alignment sensor can be rigidly and accurately mounted to the reference frame RF (fig. 1) while effectively scanning the alignment marks 202 in a direction opposite to the direction of movement of the substrate W. In this movement, the substrate W is controlled by its mounting on the substrate table WT and the substrate positioning system PW. All movements shown are parallel to the X-axis. A similar action applies to scanning the alignment mark 204 with the spot 208 in the Y direction.

As discussed in published patent application US 2012-_wThe time T is fast and available for acquiring the position of each alignment mark_ACQAnd is correspondingly short. For simplicity, the formula T applies_ACQ＝L/V_W. The previous application US 2012-. If desired, phaseThe same scanning spot technique can be applied to sensors and methods of the type disclosed herein.

Fig. 4 illustrates the optical system of the alignment sensor, which is a modified version of one of the alignment sensors described in the above-described prior publications US6,961,116 and US 2009/195768. This introduces the choice of an off-axis illumination mode that allows the pitch of the alignment marks to be reduced for higher accuracy. The optical system may also allow scatterometry type measurements to be performed with the alignment sensor rather than using a separate scatterometry instrument. In fig. 4, details of providing off-axis and on-axis modes of illumination are omitted for simplicity. It is of further interest to the present disclosure to show details of multiple wavelengths and polarizations.

The optical axis O with several branches is shown by the dashed line through the optical system shown in fig. 4. For convenience of comparison with the schematic diagram of fig. 3, certain components of the optical system shown in fig. 4 are labeled with like reference numerals as used in fig. 3, except that the prefix "4" is used instead of "2". Thus, we see a light source 420, an illumination beam 422, an objective lens 424, an information-bearing beam 426, a self-referencing interferometer 428, and a detector arrangement 430. The signals from the detector arrangement are processed by a processing unit PU which is modified in order to implement the new features described below and which outputs a (improved) position measurement POS for each alignment mark.

Additional components illustrated in this more detailed schematic are as follows. In the illumination subsystem 440, radiation from the source 420 is delivered to illumination profiling optics 446 via optical fibers 442. Which passes input beam 422 via beam splitter 454 to objective lens 424, which has a pupil plane P. The objective lens 424 forms the spot 406 on the alignment mark 202/204/210 on the wafer W. Information-bearing beam 426 diffracted by the alignment mark passes through beam splitter 454 to interferometer 428. Interferometer 428 splits the information-bearing beam into two portions with orthogonal polarizations, rotates the portions 180 ° relative to each other about the optical axis, and combines them into output beam 482. Output beam 482 enters detector arrangement 430, which is described in more detail below.

Included in this example is an asymmetry measurement arrangement 460. Arrangement 460 receives a portion 464 of information-bearing beam 426 that passes through a second beam splitter 462 positioned before the interferometer. Another patent application US 20125227061 describes a technique for asymmetry measurement using position information obtained by the detector 430. It should be understood that the asymmetry measurement arrangement 460 is optional and may therefore be omitted in other embodiments.

The illumination profiling optics 446 may take various forms, some of which are disclosed in more detail in prior patent application US 2015109624. In the example disclosed in the patent application china, an alignment sensor (more generally, a position measurement device) is shown, which allows the use of a reduced grating pitch without requiring spatial resolution on the detector side. By using multiple illumination modes, these devices are able to measure the position of the alignment marks using a wide range of different pitches, for example pitches from less than 1 μm to 20 μm, without changing the current detector design. A particular feature common to the examples described in the prior application US 2015109624 is the choice of using off-axis illumination at a limited range of angles of incidence (a limited range of radii in the pupil plane). By off-axis illumination it is meant that the source region of radiation is confined to the peripheral portion of the pupil, that is, at some distance from the optical axis. Limiting the illumination to the outermost periphery of the pupil reduces the smallest possible pitch of the alignment marks from about λ/NA to about λ/2NA, where λ is the wavelength of the radiation used and NA is the numerical aperture of the objective lens of the instrument (e.g., the alignment sensor, or more generally, the position measurement device). The example described in the prior application US 2015109624 also uses a special distribution of spot mirrors in the beam splitter of the device, which can provide both the desired illumination and the field stop for zero order diffracted radiation. A "universal" illumination profile may be designed that allows alignment on any of the X, Y and XY alignment marks without changing the illumination mode, although this will inevitably entail some compromise in performance and/or some complexity of the apparatus. Alternatively, a dedicated mode may be designed and manufactured, which may be selected for use with different european marker types. Different polarizations of illumination may also be selected.

The apparatus as a whole need not be limited to providing these particular off-axis illumination profiles. It may have other modes of use, known or yet to be developed, that support the use of different profiles. For example, the apparatus may provide for the selection of on-axis and off-axis illumination modes for the different alignment mark types shown in fig. 2(a) and 2 (b). While off-axis illumination is advantageous for use with finer gratings, on-axis illumination profiles may be usable for compatibility with existing alignment marks and measurement methods. Referring first to the example of on-axis mode, as used in the known sensor of figure 3, illumination orthogonal to the substrate is provided by an on-axis illumination profile with a central bright spot within the further dark-field pupil. The profile is an optional setting in the illumination beam 422 of the apparatus. In this example, it is not only desirable for the zero order beam returning along the optical axis to be blocked before entering the interferometer 428, but it is also desirable for it to be diverted to the asymmetry measurement arrangement 460 (when provided). Blocking the zeroth order before interferometer 428 is not necessary, but improves the signal-to-noise ratio of the position signal. Thus, in this embodiment, a spot mirror may be included in the second beam splitter 462. The first beam splitter 454 is not silvered and receives only about 50% of the intensity of the central spot that can be transferred to the alignment marks. In an alternative embodiment, where the arrangement 460 is omitted, the profile may be produced directly by the illumination profiler 446, and delivered to the objective 424 at its full intensity by the spot mirror within the first beam splitter 454. Various alternatives can be envisaged to obtain the desired profile.

The off-axis illumination profile can be generated in a variety of ways to form a practical instrument, bearing in mind that the opposing segments should be coherent for the interferometer 428 to produce the desired signal. In particular, when a broadband source is involved, the coherence length/time of the source radiation will be very short. Even if a monochromatic laser source is used, US6961116 teaches that a short coherence time is preferred, for example to eliminate interference from unwanted multiple reflections. Therefore, the optical path lengths from the source to each segment should be very closely matched. The holes directly corresponding to the desired profile may be placed in a widened parallel beam, but this results in a relatively large light loss. In order to avoid light losses, various alternative solutions are proposed in the above-mentioned prior application US 2015109624.

The illumination exiting illumination source 442 may be monochromatic in nature, but is typically broadband, such as white light, or polychromatic. It should be understood that the illumination source 442 is a source operable to emit electromagnetic radiation. The radiation may comprise electromagnetic radiation outside the visible and/or visible spectrum, for example infrared radiation. It should be understood that, hereinafter, the term "radiation" is synonymous with, and may be used interchangeably with, the term "light". Similarly, the wavelength (or range of wavelengths) of such radiation may be referred to as the "color" of the radiation, whether or not the radiation is from the visible spectrum. The diversity of wavelengths in the beam increases the robustness of the measurement, as is known. One known sensor uses, for example, a set of four wavelengths, each of which is in the range of 500nm to 900 nm. These four wavelengths may be named by their color name, which may be: green (including green), red (including red), near infrared (including radiation in the near infrared), and far infrared (including radiation in the far infrared). In a sensor embodying the invention, the same four wavelengths may be used, or different four wavelengths may be used, or more or less than four wavelengths.

Referring again to fig. 4, aspects of the device relating to the measurement of radiation using multiple wavelengths and relating to the management of polarization effects will now be described. In the illumination subsystem 440, the source 420 includes four separate light sources arranged to generate radiation having four wavelengths, which are designated green (denoted as G), red (R), near infrared (N), and far infrared (F). For the sake of the following discussion, these four different wavelengths of radiation will be referred to as four-color light, for which purpose it is immaterial whether they are in the visible or invisible portion of the electromagnetic spectrum. The light source is linearly polarized, with the G and N radiation being oriented the same as each other, and the R and F radiation being polarized orthogonal to the polarization of the G and N radiation.

The four colors are transmitted through polarization maintaining fibers to multiplexer 502 where they are combined into a single four color beam. The multiplexer maintains a linear polarization as indicated by arrow 504. Arrows 504 and similar arrows in the figure are labeled G and R to indicate the polarization of the green and red components. The N and F components are oriented identically to the G and R components, respectively.

The combined beam enters the beam splitter 454 via suitable delivery optics 506. As already described, it is then reflected from a partially or totally reflecting surface (e.g. a 0.5mm diameter spot mirror), which is internal to the beam splitter. The objective lens 424 focuses the beam into a narrow beam that is reflected and diffracted by the grating formed by the alignment marks 202 on the wafer W. The light is collected by an objective lens 424 having a numerical aperture NA of 0.6, for example. This NA value allows at least ten diffraction orders to be collected for each color from a grating having a 16 μm pitch.

The reflected and diffracted light forming information-bearing beam 426 is then directed to self-referencing interferometer 428. In this example, the beam is split by a beam splitter 462 to feed a portion 464 of the information carrying beam to the asymmetry measurement arrangement 460 (when it is set up), as already described. A signal 466 conveying asymmetry measurement information is passed from the arrangement 460 to the processing unit PU. Just before the interferometer, the polarization is rotated by 45 ° by half-wave plate 510. From this point on, the polarization arrows are shown for only one color for clarity. As already described and described in patent US6961116, the interferometer comprises a polarizing beam splitter, where half of each color is transmitted and half of each color is reflected. Each half is then reflected three times inside the interferometer, rotating the radiation field by +90 ° and-90 °, providing a relative rotation of 180 °. The two fields then overlap on top of each other and allow interference. A phase compensator 512 is provided to compensate for path differences of the-90 ° and 90 ° images. The polarization is then rotated by another half-wave plate 514 by 45 (with its major axis set to 22.5 relative to the X or Y axis). The half- wave plates 510, 514 are wavelength insensitive so that the polarization of all four wavelengths is rotated by 45.

Another beam splitter 516 splits the optical signal into two paths, labeled a and B. One path contains the sum of the two rotating fields and the other contains the difference. Depending on the initial polarization direction, the sum terminates in either path a or path B. Thus in this example the sum signal of the green and NIR signals terminates in one path and the sum signal of the red and FIR signals terminates in the other path. For each color, the corresponding difference signal terminates in another path. It is to be understood that a radiation source is a source operable to emit radiation, such as electromagnetic radiation. The radiation may comprise visible light. Alternatively, the radiation may comprise electromagnetic radiation other than in the visible spectrum, for example infrared radiation. It should be understood that in the above description, the term "radiation" is synonymous with the term "light". Any reference to light therefore includes electromagnetic radiation other than visible light.

It should be noted that this arrangement selects one polarization for illumination in each color. Two polarization measurements per color can be made by changing the polarization between readings (or by time-multiplexing in the readings). However, in order to maintain high throughput while gaining benefits from color and diversity in polarization, each color of linear polarization and one subset of colors with one polarization direction and another subset of colors with a different polarization direction represent a good compromise between diversity and measurement throughput. To increase the diversity without affecting throughput, embodiments similar to the four-color scheme presented here, but using more colors (e.g., eight or sixteen) and with mixed polarizations, are conceivable.

The light of each path a and B is collected by a respective collector lens assembly 484A and 484B. It then passes through the aperture 518A or 518B, which eliminates most of the light from outside the spot on the substrate. Two multimode fibers 520A and 520B transmit the collected light of each path to respective demultiplexers 522A and 522B. Demultiplexers 522A and 522B separate each path into the original four colors so that a total of eight optical signals are passed to detectors 430A and 430B in detector arrangement 430. In a practical embodiment, the optical fibers are arranged on eight test boards from the demultiplexer and the detectorBetween the detector elements. The detector in this example does not provide spatial resolution, but passes a time-varying intensity signal I for each color as the apparatus scans the alignment mark 202 on the substrate W_AAnd I_B. The signal is actually a position dependent signal but is received as a time varying signal (waveform) synchronized with the physical scanning motion between the device and the alignment marks (recall fig. 3).

The processing unit PU receives the intensity waveforms from the eight detectors and processes them as in the known device to provide a position measurement POS. Because there are eight signals to choose from based on different wavelengths and incident polarizations, the device can obtain usable measurements in most cases. In this regard, it should be kept in mind that the alignment marks 202 may be buried under multiple layers of different materials and structures. Some wavelengths will penetrate different materials and structures better than others. Conventionally, the processing unit PU processes the waveform and provides a position measurement value based on the one providing the strongest position signal. The remaining waveforms may be discarded. In a simplified embodiment, the "recipe" for each measurement task may specify which signal to use based on prior knowledge of the target structure and experimental investigation. In more advanced systems, the "color dynamics" or "smooth color dynamics" algorithm may be used for automatic selection to identify the best signal without prior knowledge. This is described by Jeronen Huijbregtse et al in the paper "Overlay Performance with Advanced ATHENATM Alignment Strategies, Metrology, Inspection, and Process Control for Micrographics XVII, Daniel J.Her, Editor, Proceedings of SPIE Vol.5038 (2003).

Each lens 484A, 484B focuses the entire field on each element of each detector 430A, 430B, each detector 430A, 430B being a similar arrangement to the known alignment sensor of fig. 3. The detector in this example and in the known alignment sensor is effectively a single photodiode and does not provide any spatial information other than by the scanning motion already described. If desired, detectors with spatial resolution can be added in the conjugate pupil plane. This may for example allow for angle-resolved scattering methods by using quasi-sensor hardware.

If, for example, it is desired to use two different polarization measurement positions, the alignment marks may need to be scanned more than once. It may also be necessary to switch the irradiation mode in the middle of scanning the XY alignment mark. However, in other embodiments, multiplexing of the optical signals is used so that two measurements can be made simultaneously. Similarly, multiplexing techniques may be applied so that different portions of the XY alignment mark may be scanned and measured without switching the illumination mode. One simple way to implement this multiplexing technique is by frequency division multiplexing. In this technique, the radiation from each pair of spots and/or polarizations is modulated using a characteristic frequency, selected to be well above the frequency of the time varying signal carrying the position information. The diffracted and processed optical signals arriving at each detector 430A, 430B will be a mixture of the two signals, but they may be separated electrically using filters tuned to the respective frequencies of the source radiation. Time division multiplexing may also be used, but this would require accurate synchronization between the source and detector. The modulation at each frequency may be, for example, a simple sine wave or a square wave.

If it is desired to illuminate the alignment marks with circular polarization, a quarter-wave plate (not shown) may be inserted between beam splitter 454 and objective lens 424, whether for position sensing or some other form of measurement. This has the effect of changing the linear polarization into a circular polarization (and changing it back again after diffraction by the alignment marks). The spot position is selected as before depending on the orientation of the alignment marks. The direction of circular polarization (clockwise/counterclockwise) can be changed by selecting different linear polarizations in the illumination source 420, the optical fiber 422, or the illumination profiling optics 446.

The use of multiple gratings in a composite target is also described in the paper by Huijbregtse et al. Each grating has a different profile, for example enhanced by higher diffraction orders (third, fifth, seventh). Position measurements may be obtained from different ones of these gratings, as well as different color signals on the individual gratings. In this disclosure, a single grating with a simple grating pattern, but with segmented features, is assumed. The skilled person can easily extend the disclosure to envisage embodiments with a plurality of gratings with different patterns.

FIG. 5 is a schematic diagram of a supercontinuum radiation source 600 according to an embodiment of the present invention. Supercontinuum radiation source 600 includes radiation source 610, illumination optics 620, a plurality of waveguides 630a-630n, and collection optics 640.

The radiation source 610 is operable to generate a pulsed radiation beam. In the following, the radiation source 610 may be referred to as pump radiation source 610 and the pulsed radiation beam 612 may be referred to as pump radiation beam 612. It should be appreciated that the pulsed radiation beam 612 includes a plurality of sequential, discrete, and temporally separated radiation pulses. The pulsed radiation beam may typically have a substantially constant pulse frequency, which may be of the order of 20-80 MHz. The pump radiation source 610 may comprise a laser. For example, the laser may comprise a mode-locked laser. Suitable lasers may include fiber lasers such as, for example, ytterbium (Yb-doped) fiber lasers. Other suitable lasers may include titanium sapphire (Ti: sapphire) lasers. Each radiation pulse may have a duration in the order of 0.1-1 picosecond.

The illumination optics 620 is arranged to receive the pulsed pump radiation beam 612 and form a plurality of pulsed beamlets 622a-622 n. Each pulsed beamlet 622a-622n comprises a portion of the pulsed pump radiation beam 612. Each of the plurality of waveguides 630a-630n is arranged to receive at least one of the plurality of pulsed sub-beams 622a-622 n. In the embodiment shown in FIG. 5, each of the plurality of waveguides 630a-630n is arranged to receive a different one of the plurality of pulsed beamlets 622a-622 n. For example, a first waveguide 630a of the plurality of waveguides receives a first pulsed beamlet 622a of the plurality of pulsed beamlets, and a second waveguide 630b of the plurality of waveguides receives a second pulsed beamlet 622b of the plurality of pulsed beamlets. In this manner, the pulsed pump radiation beam 612 is passively divided into multiple portions, each portion being received by one of the plurality of waveguides, and the power of the pulsed pump radiation beam 612 is distributed across the plurality of waveguides.

Each of the plurality of waveguides 630a-630n is arranged such that as its corresponding pulsed sub-beam 622a-622n propagates through the waveguides 630a-630n, the spectrum of that pulsed sub-beam 622a-622n is broadened, thereby generating a supercontinuum sub-beam 632a-632 n. Accordingly, supercontinuum radiation source 600 includes a plurality of waveguides 630a-630n having suitable nonlinear optical properties to allow supercontinuum generation in each of the plurality of waveguides 630a-630 n.

It should be understood that the term "waveguide" as used herein refers to a structure configured to guide waves, particularly electromagnetic waves. Such a waveguide may form part of an integrated optical system, i.e. it may be provided "on-chip". Alternatively, such a waveguide may be a free space waveguide. Free space waveguides include a variety of different types of optical fibers including, for example, photonic crystal fibers.

Collection optics 640 is arranged to receive supercontinuum sub-beams 632a-632n from multiple waveguides 630a-630n and combine them to form supercontinuum radiation beam B_outWhich is output by the supercontinuum radiation source 600.

The supercontinuum radiation source 600 may be particularly suitable for use within an alignment mark measurement system. For example, the supercontinuum radiation source 600 may correspond to the illumination sources 220, 420 shown in FIGS. 2 and 4, respectively, and the supercontinuum radiation beam B_outMay correspond to radiation beams 222, 422. However, the supercontinuum radiation source 600 may also be suitable for use within other optical measurement systems, for example in semiconductor inspection equipment. Other examples of applications of the supercontinuum radiation source 600 are fiber optic inspection, interferometry or spectroscopy, medical applications such as optical coherence tomography, confocal microscopy, etc.

The supercontinuum radiation source 600 may be operated to produce a supercontinuum radiation beam B having a relatively broad spectrum_out. For example, supercontinuum radiation beam B_outMay have a spectrum extending from the visible range to the far infrared, for example the spectrum may extend from 400nm to 2500 nm. Such a radiation beam B_outThis is particularly useful for alignment mark measurement systems, such as the alignment sensors shown in fig. 3 and 4.

One advantage of using multiple waveguides 630a-630n for supercontinuum generation is that the supercontinuum radiation source 600 has a level of redundancy and can still operate to some extent even in the event of a failure of one of the multiple waveguides 630a-630 n.

When a radiation pulse propagates through a waveguide, a super-continuity is formed due to various nonlinear optical effects. Due to the inherent non-linear nature of these effects, supercontinuum radiation sources are typically subject to spectral noise, pulse-to-pulse fluctuations, and fluctuations in output mode, even though the pump radiation source generating these pulses is stable such that its output radiation beam has substantially no pulse-to-pulse variations.

Supercontinuum radiation source 600 provides an arrangement in which a plurality of supercontinuums 632a-632n (one for each of a plurality of waveguides 630a-630 n) are generated and superimposed (by collection optics 640). This arrangement is advantageous over prior art arrangements because noise and pulse-to-pulse variations within different individual supercontinuums 632a-632n will at least partially cancel each other out. Thus, the arrangement provides a broad spectrum radiation source of the type suitable for use in an alignment mark measurement system, which has a more stable output than prior art arrangements.

In general, a waveguide will be able to support radiation provided that the radiation intensity (i.e. power per unit area) is below the threshold of the waveguide. If radiation having an intensity above the threshold is coupled into the waveguide, the waveguide may be damaged. The supercontinuum radiation source 600 allows the power of the pulsed pump radiation beam 612 to be distributed over a plurality of waveguides 630a-630n by splitting the pulsed pump radiation beam 612 into a plurality of sub-beams 622a-622n, each propagating through a different waveguide to produce a supercontinuum. This means that for a given desired output power of the source, the cross-sectional area of each of the plurality of waveguides 630a-630n can be reduced relative to the cross-sectional area of a single waveguide in a prior art supercontinuum source.

It may be desirable to provide a broadband radiation source with a relatively high output power, for example an output power in the order of 1W. Known supercontinuum radiation sources with output powers of this order are feasible by using, for example, photonic crystal fibers as the nonlinear optical medium.

In some embodiments of the present invention, even for relatively bright supercontinuum radiation sources 600 (e.g., with power on the order of 1W or more), the size of the waveguide may be sufficiently reduced so that the waveguide may include integrated optics. That is, the waveguide may be disposed on a chip (e.g., as an integrated optical system) and may be formed using semiconductor fabrication techniques. For example, the supercontinuum radiation source 600 may have an output power on the order of 1W. To achieve this, the pump radiation source 610 may provide an input power of about 2-10W or slightly more to account for power losses through the supercontinuum radiation source 600. The supercontinuum radiation source 600 may include on the order of 1000 waveguides 630a-630n, such that each waveguide 630a-630n supports a beamlet 622a-622n having a power on the order of 1mW (e.g., in the range of 2-10 mW). A radiation beam having a power on the order of about 1mW may allow the size of the waveguides 630a-630n to be sufficiently reduced such that the waveguides 630a-630n may include integrated optics.

Fig. 6a shows a cross-sectional view of a portion of a waveguide 650 provided as part of an integrated optical system 651 in a plane (the x-y plane) perpendicular to the optical axis of the waveguide 650 (i.e. in a plane perpendicular to the direction in which radiation propagates through the waveguide 650 in use). Any or all of the plurality of waveguides 630a-630n shown in FIG. 5 may generally be in the form of a waveguide 650.

Waveguide 650 is made of a suitable nonlinear optical material (e.g., silicon nitride (Si)₃N₄) Formed of a cladding material 652, such as, for example, silicon or silicon dioxide (SiO)₂) ) is surrounded.

It will be appreciated that the waveguide 650 generally extends in a direction perpendicular to the plane of fig. 6a (i.e. the z-direction). The cross-sectional shape and size of the waveguide 650 is substantially constant along the extent in the z-direction. To illustrate this, fig. 6b shows a partial cross-sectional view of a portion of waveguide 650 shown in fig. 6a, in which cladding material 652 is not shown.

The waveguide 650 may have a width 653 on the order of 1 μm or less and may have a height 654 on the order of several hundred nm. The waveguide may extend in the z-direction over a distance in the order of a few millimeters.

Waveguide 650 may be formed, at least in part, using semiconductor fabrication techniques. For example, a resist layer may be applied to the substrate formed of the cladding material 652 and the resist may be patterned using photolithographic techniques. This may then be used to selectively etch a trench into cladding material 652 to accommodate waveguide 650. Note that the resist may be patterned such that multiple trenches (each for accommodating a different waveguide) may be formed simultaneously during the etching process. For example, the plurality of waveguides 630a-630n of the supercontinuum radiation source may be formed on a common substrate. The waveguide 650 may be formed by depositing a material (e.g., silicon nitride) into a trench formed during an etching process. For example, silicon nitride may be deposited, for example using a Low Pressure Chemical Vapor Deposition (LPCVD) technique, to form a high quality solid waveguide 650 from the silicon nitride. Finally, a layer of cladding material 652 may be applied over waveguide 650 and surrounding cladding material 652 to completely enclose waveguide 650 within cladding material 652.

On-chip waveguides (e.g., the form of waveguide 650 shown in fig. 6a and 6 b) typically have a small interaction length over which the nonlinear processes that result in supercontinuum generation can work, rather than, for example, free-space waveguides (e.g., photonic crystal fibers) that are used to generate supercontinuum. For example, a waveguide on a chip (e.g., in the form of waveguide 650 shown in fig. 6a and 6 b) may have a length of 10mm or less.

This, in turn, reduces the noise and pulse-to-pulse variation of the supercontinuum 632a-632n produced by each of the plurality of waveguides 630a-630n, relative to the noise and pulse-to-pulse variation of a single waveguide in prior art supercontinuum sources.

Thus, the supercontinuum radiation source 600 allows for dual improvements in noise and pulse-to-pulse variation in output. Each individual supercontinuum 632a-632n may be generated with a more stable output (for a given total output power) than prior art arrangements, and in addition, multiple supercontinuums 632a-632n are combined to at least partially average noise and pulse-to-pulse fluctuations.

Furthermore, the properties of the waveguide on the chip allow the supercontinuum radiation source 600 to benefit from the supercontinuum radiation beam B output by the supercontinuum radiation source 600_outBetter mode control and polarization control. The attributes of the on-chip waveguide that provide these benefits include: the relatively short interaction length of the waveguides on the chip, the refractive index contrast of the integrated optical device (between the waveguide material and the cladding material), and the fabrication technique of the waveguides on the chip.

The illumination optics 620 and collection optics 640 may take any of a number of different forms, as now discussed with reference to fig. 7, 8a, 8b, 9a, and 9 b.

FIG. 7 shows a first embodiment 700 of the supercontinuum radiation source 600 of FIG. 5. In the embodiment shown in FIG. 7, by way of example, only four waveguides 630a-630d are shown that produce a supercontinuum. In this first embodiment, both the illumination optics 620 and the collection optics 640 are implemented by a waveguide system (which may be an integrated optic or an optical fiber).

The illumination optics 620 includes a primary waveguide 721, two secondary waveguides 722, 723 and four tertiary waveguides 724-727. The primary waveguide 721 receives the pump radiation beam 612 and is coupled to the two secondary waveguides 722, 723, for example such that each of the two second waveguides 722, 723 receives half the power of the pump radiation beam 612. In turn, each of the two secondary waveguides 722, 723 is coupled to two of the tertiary waveguides 724, 725 and 726, 727, respectively, for example such that each of the four tertiary waveguides 724-727 receives a quarter power of the pump radiation beam 612. Each of the four tertiary waveguides 724-727 is coupled to a different one of the four waveguides 630a-630d that produce the supercontinuum.

The illumination optics 620 shown in fig. 7 includes multiple levels (horizontal) or segments of waveguides, each level or segment having twice the number of waveguides as the previous level. For example, the primary waveguide 721 may be considered a first stage, the secondary waveguides 722, 723 may be considered a second stage, and the tertiary waveguides 724-727 may be considered a third stage. As described above, in the embodiment shown in FIG. 7, only four waveguides 630a-630d are shown that produce a supercontinuum, and thus the illumination optics include three stages.

In this exemplary embodiment, the radiation within each waveguide of a given stage of illumination optics 620 is coupled to two waveguides on the next stage such that each of the two waveguides on the next stage receives half the power of the radiation. It should be understood that the radiation may be divided between the two waveguides on the next stage in a variety of different ways. For example, different portions of the power of radiation from a waveguide on a particular level may be coupled into each waveguide on the next level. Further, in alternative embodiments, the radiation within each waveguide of a given stage may be coupled to more than two waveguides on the next stage, such that each of the more than two waveguides on the next stage receives a desired fraction of the power of the radiation.

Collection optics 640 includes four primary waveguides 741-744, two secondary waveguides 745, 746, and one tertiary waveguide 747. Each of the primary waveguides 741-744 receives a different one of the supercontinuum sub-beams 632a-632d output by one of the waveguides 630a-630 d. Each of the two secondary waveguides 745, 746 is coupled to two of the primary waveguides 741, 742 and 743, 744, respectively, such that each of the two secondary waveguides 745, 746 receives and combines two of the supercontinuum sub-beams 632a-632 d. The tertiary waveguide 747 is coupled to the two secondary waveguides 745, 746 such that the tertiary waveguide 747 receives and combines all four supercontinuum sub-beams 632a-632 d.

The collection optic 640 shown in fig. 7 includes multiple stages of waveguides, each stage having as many waveguides as half of the waveguides of the previous stage. For example, the primary waveguide 741-744 can be considered a first stage, the secondary waveguides 745, 746 can be considered a second stage, and the tertiary waveguide 747 can be considered a third stage. As described above, in the embodiment shown in FIG. 7, only four waveguides 630a-630d are shown that produce a supercontinuum, and thus the collection optics includes three stages.

In this exemplary embodiment, radiation within each waveguide of a given stage of collection optics 640 is coupled to two waveguides on the next stage such that each of the two waveguides on the next stage receives half the power of the radiation. It should be understood that the radiation may be divided between the two waveguides on the next stage in a variety of different ways. For example, different portions of the power of radiation from a waveguide of a particular stage may be coupled into each waveguide on the next stage. Further, in alternative embodiments, the radiation within each waveguide of a given stage may be coupled to more than two waveguides on the next stage, such that each of the more than two waveguides on the next stage receives a desired fraction or proportion of the power of the radiation.

In the embodiment shown in FIG. 7, only four waveguides 630a-630d are shown that produce a supercontinuum. It will of course be appreciated that this arrangement can be extended to include more than four waveguides producing supercontinuum. It will be apparent to those skilled in the art that this expansion can be achieved by providing illumination optics and collection optics of the same general form as shown in figure 7, but each having more than three stages of waveguides.

The waveguides 721, 727, 741, 747 that make up the illumination optics 620 and collection optics 640 may be single mode or multi-mode waveguides.

Furthermore, the waveguides 721, 727, 741, 747 that make up the illumination optics 620 and collection optics 640 may include integrating optics, free-space optics, or a combination of both. As described above, in some embodiments, the supercontinuum radiation source 600 may include on the order of 1000 waveguides 630a-630 n. Each of these waveguides 630a-630n may support a beamlet 622a-622n with a power of the order of 1mW, for example. For such embodiments, the general form of illumination optics and collection optics shown in fig. 7 may be expanded such that there are more than three levels of waveguides. For example, each illumination optic and collection optic may include 11 stages of waveguides, each stage differing from the previous stage by a factor of 2. Such an arrangement allows for the presence of 1024(210) waveguides 630a-630n that produce a supercontinuum. In general, multiple levels of waveguides with sufficiently low power in the illumination optics 620 and collection optics 640 may be implemented as integrated optics. For example, the multiple levels of waveguides closest to the waveguides 630a-630n that produce the supercontinuum may be implemented as integrated optics. In general, the multiple stages of waveguides in illumination optics 620 and collection optics 640 that have sufficiently high power to make them impractical as integrated optics may be implemented as free-space optics. For example, the multiple levels of waveguides furthest from the waveguides 630a-630n that produce the supercontinuum may be implemented as integrated optics.

As discussed above, in alternative embodiments, radiation within each waveguide of a given stage may be coupled to more than two waveguides on the next stage, such that each of the more than two waveguides on the next stage receives a desired proportion of the power of the radiation. In one embodiment, the illumination optics 620 and collection optics 640 may be implemented such that a single waveguide is coupled to each of the waveguides 630a-630n that produce the supercontinuum. However, the above-described arrangement in which each of the illumination optics 620 and collection optics 640 includes a plurality of different stages may be more readily implemented, particularly for embodiments including a large number of waveguides 630a-630n (e.g., on the order of 1000) that produce a supercontinuum.

FIGS. 8a and 8b show two variations of the second embodiment 800 of the supercontinuum radiation source 600 of FIG. 5.

In the embodiment 800 shown in FIGS. 8a and 8b, only four waveguides 630a-630d are shown that produce a supercontinuum. As with the embodiment 700 shown in fig. 7, this is merely to provide a specific illustrative example of this embodiment 800, and it will be clear or obvious to the skilled person how the embodiment 800 of fig. 8a, 8b can be extended to embodiments having more (or less) than four waveguides producing supercontinuum. In this second embodiment, both the illumination optics 620 and the collection optics 640 are implemented by lens systems.

The illumination optics 620 in fig. 8a and 8b comprises collimating optics 821 and focusing optics 822.

The collimating optics 821 comprises a focusing lens (e.g., a convex lens) arranged to receive the pump radiation beam 612 from the pump radiation source 610, collimate the pump radiation beam 612 and direct it onto the focusing optics 822. In particular, the collimating optics 821 may be arranged to substantially uniformly illuminate the focusing optics 822.

The focusing optics 822 are arranged to optically couple different portions of the pump radiation beam 612 to each of the four supercontinuum generating waveguides 630a-630 d. In particular, the focusing optics 822 are arranged to focus different portions of the pump radiation beam 612 to a focal point at or near the entrance of each of the four supercontinuum producing waveguides 630a-630 d. To achieve this, the focusing optics 822 comprise an array of focusing lenses 822a-822d, each arranged to focus a different portion of the pump radiation beam 612 to a focal point at or near the entrance of each of the four supercontinuum producing waveguides 630a-630 d. The focusing lens array 822 may comprise a one-dimensional array or a two-dimensional array. Each focusing lens 822a-822d may comprise, for example, a spherical lens. Each individual focusing lens 822a-822d may include a microlens. The microlenses may be lenses having a diameter of less than 1 mm.

In general, the focusing optics 822 may be implemented using integrated optics or free space optics. It should be understood that the above-described embodiment in which the focusing optics 822 comprise an array of focusing microlenses 822a-822d is only one example of suitable focusing optics 822. The array of focusing micro-lenses 822a-822d can be implemented relatively simply. Since the pump radiation beam 612 may be monochromatic, in an alternative embodiment the focusing optics 822 may comprise an optical system arranged to produce a diffraction pattern comprising an array of maxima, each maximum coinciding with one of the waveguides 630a-630d producing the supercontinuum. These maxima may each comprise a different portion of the pump radiation beam 612 and may be focused to a focal point at or near the entrance of each of the four supercontinuum producing waveguides 630a-630 d. Suitable optical systems for implementing such focusing optics 822 may include one or more of the following: a diffraction grating, a Spatial Light Modulator (SLM), or a fresnel lens array.

The collection optics 640 in fig. 8a and 8b include collimating optics 841 and focusing optics 842.

The collimating optics 841 includes a focusing lens (e.g., a convex lens) that is arranged to receive and collimate the supercontinuum sub-beams 632a-632n from each of the plurality of waveguides 630a-630 n. In alternative embodiments, collimating optics 841 may include any multi-color focusing optics, including concave mirrors. Collimating optics 841 is arranged such that supercontinuum sub-beams 632a-632n so collimated are spatially adjacent such that they are effectively combined to form supercontinuum radiation beam B_outWhich is output by the supercontinuum radiation source 800.

Collimating optics 841 is arranged to direct supercontinuum radiation beam B_outOnto the focusing optics 842. In turn, the focusing optics 842 are arranged to direct the supercontinuum radiation beam B_outOptically coupled to an optical fiber 850 that can deliver a supercontinuum radiation beam B_outDirected to using a supercontinuum radiation beam B_outSuch as an alignment sensor. The focusing optics 842 include a focusing lens, such as a convex lens.

In the embodiment 800 shown in FIGS. 8a and 8b, only four waveguides 630a-630d are shown that produce a supercontinuum. It will of course be appreciated that this arrangement can be extended to include more than four waveguides producing supercontinuum.

The optical fiber 850 may be a single mode or multimode waveguide. In the arrangement shown in fig. 8a, the focusing optics 842 are coupled directly into the optical fiber 850. This arrangement may be suitable for use with a supercontinuum radiation beam B_outThe multi-mode output of (1). In the arrangement shown in fig. 8b, the focusing optics 842 are coupled into the optical fiber 850 via a pinhole aperture (pinhole aperture) 851. This arrangement may be suitable for use with a supercontinuum radiation beam B_outThe single mode output of (2). Pinhole aperture 851 acts as a spatial filter. Such a pinhole aperture or a single mode waveguide may be used as a wavefront filter. Pinhole aperture 851 may be arranged to eliminate high spatial frequency wavefront defects of the focused wave. Advantageously, the attenuation of such pinhole aperture 851 is relatively independent of the wavelength of the radiation, such that the pinhole aperture 851 does not significantly affect the supercontinuum radiation beam B_outThe overall shape of the spectrum of (a).

FIGS. 9a and 9b show two variations of the third embodiment 900 of the supercontinuum radiation source 600 of FIG. 5.

In the embodiment 900 shown in FIGS. 9a, 9b, only four waveguides 630a-630d are shown that produce a supercontinuum. As with the embodiment 700 shown in fig. 7, this is merely to provide a specific illustrative example of this embodiment 900, and it will be apparent to the skilled person how the embodiment 900 of fig. 9a, 9b can be extended to embodiments having more (or less) than four waveguides producing supercontinuum. In this second embodiment, both the illumination optics 620 and the collection optics 640 are implemented by optical fibers or waveguide systems.

Illumination optics 620 typically has the form shown in fig. 7 as described above and will not be described further herein.

Collection optics 640 includes four lensed fibers 941 and 944. Each lensed fiber 941-944 (also referred to as a tapered fiber) is coupled to a different one of the waveguides 630a-630d that generates the supercontinuum such that it receives the supercontinuum sub-beams 632a-632d output by that waveguide 630a-630 d. This arrangement is advantageous because the coupling between such lensed fibers 941-. Lensed fibers 941-.

The four lensed fibers 941-. This may be accomplished using a coupling 945 that may include, for example, one or more fused fiber optic couplings.

Coupling 945 is arranged such that supercontinuum sub-beams 632a-632d are effectively combined to form supercontinuum radiation beam B_outWhich is output by the supercontinuum radiation source 900 via output optical fiber 950.

In the embodiment 900 shown in FIGS. 9a and 9b, only four waveguides 630a-630d are shown that produce a supercontinuum. It will of course be appreciated that this arrangement can be extended to include more than four waveguides producing supercontinuum.

The optical fiber 950 may be a single mode or multimode waveguide. In the arrangement shown in FIG. 9a, the output optical fiber 950 is adapted to output a multimode supercontinuum radiation beam B_outThe multimode optical fiber of (1). In the arrangement shown in fig. 9b, after the coupling 945, the output fibre 950 is provided with a pinhole aperture 951. In this manner, the output from the (multimode) fiber 950 can be spatially filtered through the pinhole aperture 951. Pinhole aperture 951 may be disposed near an end of optical fiber 950, and supercontinuum beamlets 632a-632d propagating from four lensed optical fibers 941 and 944 may be focused at or near pinhole aperture 951. This may be accomplished, for example, using focusing optics (e.g., lenses) disposed between lensed fibers 941 and 944 and pinhole aperture 951 to couple light to optical fiber 950. This arrangement may be suitable for use with a supercontinuum radiation beam B_outThe single mode output of (2).

In some embodiments of the present invention, collection optics 640 may include one or more light mixing rods. The light mixing rod may for example generally have the form disclosed in published patent applications WO2013/088295 and WO 2013/114259. For example, the light mixing rod may comprise one of the products sold by the optical solutions group of new thinking technology corporation (Synopsys), edmons Optics Inc (edmungton USA) of barreton (barrington USA) or the Lighting and Imaging department of schottky Inc (SCHOTT AG Lighting and Imaging) of germany. The light mixing rod is an extruded rod that uses multiple internal reflections to homogenize the light beam. A typical light mixing rod is made of transparent plastic, on the order of 1 cm in diameter and 10 cm in length. The non-uniform light beam enters the mixing rod at one of its ends, totally internally reflects (multiple times) along the length of the mixing rod and exits from the opposite end of the mixing rod. The emitted beam is generally more uniform than the incident beam. The mixing performance depends to a large extent on the cross-sectional shape of the mixing rod. Conventional mixing rods have, for example, a circular, square or hexagonal shape and result in a general degree of mixing. The light mixing rod may be, for example, an elongated element comprising a cross-sectional shape (transverse to its length) that is neither circular nor regular polygonal, but rather, for example, a chaotic billiard cross-sectional shape. As disclosed in published patent applications WO2013/088295 and WO2013/114259, these mixing rods result in a significantly enhanced homogenization of the incoherent light beam.

A light mixing rod is an extruded rod that uses multiple internal reflections to homogenize the light beam. These known optical elements are suitable for combining and/or mixing a plurality of input beams and forming a homogeneous output beam. Since the optical mixing rods comprise scattering material, they may cause temporal broadening of the radiation pulses output by the supercontinuum radiation source 600. A typical light mixing rod is made of transparent plastic, on the order of 1 cm in diameter and 10 cm in length. The inhomogeneous beam entering the mixing rod at one end is totally internally reflected a number of times along the mixing rod and leaves the mixing rod at the opposite end of the mixing rod. The beam profile leaving the light mixing rod is generally more uniform than the incident beam. The mixing performance depends on the cross-sectional shape of the mixing rod. The cross-section of the light mixing rod may be, for example, circular, square or hexagonal. This arrangement may result in a general degree of mixing. For example, a higher level of light mixing may be achieved using a chaotic billiard cross-sectional shape. Such a mixing rod (using a chaotic billiard cross-sectional shape) results in significantly enhanced homogenization of the incoherent light beam.

The arrangements 700, 800, 900 shown in fig. 7, 8a, 8b, 9a and 9b show examples of illumination optics 620 and collection optics 640, the illumination optics 620 providing passive coupling from the pump radiation source 610 to the waveguides 630a-630n that produce the supercontinuum; collection optics 640 provide coupling between the waveguides 630a-630n that produce the supercontinuum and the output of the supercontinuum radiation source 600. It should be understood that illumination optics 620 and collection optics 640 are not limited to these embodiments, and that one or both may be implemented with alternative optical couplings. In particular, two or more of the optical couplings described above in fig. 7, 8a, 8b, 9a and 9b may be combined as desired.

It should be understood that a radiation source is a source operable to emit radiation, such as electromagnetic radiation. The radiation may comprise visible light. Thus, it should be understood that the term "radiation" may be synonymous with the term "light".

Although specific reference has been made to an alignment method using a position measurement device, it will be appreciated that the method of measuring asymmetry described herein may also be used to measure overlay between layers of a substrate. This method may be applied, for example, when measuring the overlap between coarse and fine features of different layers of a substrate.

Although embodiments of the radiation source according to the present invention are specifically mentioned herein in the context of an alignment measurement system, embodiments of the radiation source may also be advantageously used in other optical measurement systems, for example, typically in semiconductor inspection equipment. Furthermore, the radiation source according to the invention is a supercontinuum source generating a broad spectrum with relatively low noise, which can be advantageously applied in other technical fields than lithography, such as, for example, medical tomography, measurement of optical fiber or component attenuation, interferometry or spectroscopy, optical coherence tomography, confocal microscopy, nanotechnology, biomedicine, consumer electronics, etc.

Although specific reference may have been made herein to embodiments of the invention in the context of lithographic apparatus, embodiments of the invention may be used in other apparatus. A radiation source according to an embodiment of the invention may for example be used for medical applications, for example as part of a metrology system within a medical apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or a mask (or other patterning device). These devices may be generally referred to as lithographic tools. Such a lithography tool may use vacuum conditions or ambient (non-vacuum) conditions.

It will be appreciated that the processing unit PU, which controls the alignment sensor, processes the signals detected by it and calculates from these signal position measurements suitable for use in controlling the lithographic patterning process, will typically comprise some kind of computer component, which will not be described in detail. The computer component may be a dedicated computer outside the lithographic apparatus, it may be a processing unit or a unit dedicated to the alignment sensor, or alternatively it may be a central control unit LACU controlling the lithographic apparatus as a whole. The computer component may be arranged for loading a computer program product comprising computer executable code. When the computer program product is downloaded, this may cause the computer assembly to control the aforementioned use of the lithographic apparatus with the alignment sensor AS.

Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of integrated circuits, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. Those skilled in the art will appreciate that, in the context of such alternative applications, any use of the terms "wafer" or "die" herein may be considered as synonymous with the more general terms "substrate" or "target portion", respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. In addition, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

Although specific reference may have been made above to the use of embodiments in the context of optical lithography, it will be appreciated that the embodiments may be used in other applications, for example imprint lithography, and where the context allows, are not limited to lithography. In imprint lithography, a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist provided to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. After the resist is cured, the patterning device is moved out of the resist, leaving a pattern therein.

The terms "radiation" and "beam" as used herein encompass all types of electromagnetic radiation, including infrared radiation (e.g. having a wavelength of 800nm-2.5 μm), visible radiation(e.g. having a wavelength of 380nm-800 nm), Ultraviolet (UV) radiation (e.g. having a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm). In the context of exposing a substrate, for example, using the lithographic apparatus shown in FIG. 1A, the terms "radiation" and "beam" may include: ultraviolet (UV) radiation (e.g. having a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams, output a supercontinuum radiation beam B from a supercontinuum radiation source 600 as shown in FIG. 5_outIn the context of (1), the terms "radiation" and "beam" may include: infrared radiation (e.g., having a wavelength of 800nm-2.5 μm) and visible radiation (e.g., having a wavelength of 380nm-800 nm).

The term "lens", where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. For example, the invention may take the form of a computer program containing one or more sequences of machine-readable instructions describing a method as disclosed above, or a data storage medium (e.g. semiconductor memory, magnetic or optical disk) having such a computer program stored therein.

The above description is intended to be illustrative, and not restrictive. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims (15)

1. A supercontinuum radiation source comprising:

illumination optics arranged to receive a pulsed pump radiation beam having a power and form a plurality of pulsed sub-beams, each pulsed sub-beam comprising a portion of the pulsed pump radiation beam;

a plurality of waveguides, each waveguide arranged to receive at least one of a plurality of pulsed beamlets and broaden the spectrum of the pulsed beamlet to produce a supercontinuum beamlet, wherein the power of the pulsed pump radiation beam is spread across the plurality of waveguides; and

collection optics arranged to receive the supercontinuum sub-beams from the plurality of waveguides and combine them to form a supercontinuum radiation beam,

wherein the pulsed pump radiation beam is passively coupled into and through a plurality of waveguides without applying any amplification.

2. The supercontinuum radiation source of claim 1, wherein the plurality of waveguides includes integrated optics.

3. The supercontinuum radiation source of claim 2, wherein the plurality of waveguides are formed of silicon nitride and surrounded by cladding material.

4. The supercontinuum radiation source of claim 2, wherein the plurality of waveguides are formed of silicon nitride and are silicon or silicon dioxide (SiO)₂) And (4) surrounding.

5. The supercontinuum radiation source of claim 2, wherein the plurality of waveguides are formed on a common substrate.

6. The supercontinuum radiation source of claim 2, wherein the plurality of waveguides are 1 μm or less in width and 500nm or less in height.

7. The supercontinuum radiation source of claim 2, wherein each of the plurality of waveguides has a length of 10mm or less.

8. The supercontinuum radiation source of any one of claims 1-7, wherein the supercontinuum radiation beam has a spectrum comprising radiation in the wavelength range of 400 to 2600 nm.

9. The supercontinuum radiation source of any one of claims 1-7, comprising 100 or more waveguides.

10. The supercontinuum radiation source of any one of claims 1-7, wherein the illumination optics and/or the collection optics comprise a plurality of sets of waveguides, the plurality of sets of waveguides being ordered in a sequence, and wherein a waveguide from each set of waveguides is optically coupled to a plurality of waveguides in a next set of waveguides in the sequence.

11. The supercontinuum radiation source of any one of claims 1-7, wherein the illumination optics and/or collection optics comprise a plurality of lensed fibers, each of the plurality of lensed fibers coupled to at least one of the plurality of waveguides.

12. The supercontinuum radiation source of any one of claims 1-7, wherein the illumination optics comprise first optics and focusing optics, wherein the first optics are arranged to receive a radiation beam from the radiation source and direct the radiation beam onto the focusing optics, and wherein the focusing optics are arranged to optically couple different portions of the pump radiation beam to at least two of the plurality of waveguides.

13. An optical measurement system comprising a supercontinuum radiation source according to any one of the preceding claims.

14. An alignment mark measurement system comprising:

the supercontinuum radiation source of any one of claims 1 to 12;

an optical system operable to project the supercontinuum radiation beam onto an alignment mark on a substrate supported on a substrate table;

a sensor operable to detect radiation diffracted/scattered by an alignment mark and to output a signal containing information relating to the position of the alignment mark; and

a processor configured to receive signals from the sensors and to determine a position of the alignment mark relative to the substrate table in dependence on the signals.

15. A lithographic apparatus comprising an alignment mark measurement system according to claim 14.

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